Immune system, stem cells pique researcher’s interest

Although her undergraduate degree is in mathematics, Brenda Ogle pursued advanced degrees in engineering because she hoped to apply engineering principles to the medical field. “You can see almost instantaneous application to improving life,” she says, “which is really what my whole career is about.”

An established cardiovascular biomechanics researcher whose interests include studying stem cell differentiation for cardiovascular regenerative therapies, Ogle joined the Department of Biomedical Engineering in August.

As a Ph.D. student at the University of Minnesota, Ogle was part of a group developing a tissue-engineered blood vessel — the kind of structure that could replace clogged small-diameter vessels during repair procedures. “The idea was simple,” she says. “We seeded human muscle cells into a collagen matrix.”

However, the existing construct didn’t hold up to even low pressures, so Ogle added an endothelial layer so that blood wouldn’t clot on the inside of the vessel. She also used some paracrine factors, including vitamin C and retinoic acid, to stimulate the smooth muscle cells to make their own collagen and elastin network to improve the vessel biomechanics.

When she left the University of Minnesota to become a postdoctoral fellow, and later an assistant professor, at the Mayo Clinic College of Medicine, the vessel biomechanics were similar to those of a native vessel. Now, researchers are testing the vessels in some preclinical trials, and Ogle hopes to collaborate with Minneapolis-based medical-device manufacturer Enduratec to pursue ways to integrate stem cells to further enhance vessel properties.

At Mayo, Ogle joined a group of researchers who were studying transplant biology. “Though I didn’t want to leave the biomechanics side of the research that I’d been doing, I felt the need to understand how the immune response would respond to the cellular constructs that we were developing,” she says.

She began working on projects related to driving differentiation of stem cells. In particular, she studied how to control differentiation of human T-cells — those lymphocytes that are among the body’s primary cellular responders against mainly viral pathogens — from hematopoietic stem cells, which form all the types of blood cells in the body.

To start, the group developed a model, using a pig as a surrogate host for developing human T-cells. “We would first harvest the human stem cells and inject them into fetal animals,” says Ogle. “They had to be time-pregnant animals, and the surgical component in and of itself took six to eight months to develop.”

During that time, Ogle perfected a method for determining how many different T-cells are present in a person’s body. The method uses T-cell receptor RNA to identify T-cell clones; each clone has a different receptor, or protein, that recognizes viruses or parts of viruses. When Ogle applied to a gene chip a sample with more diversity, or a greater variety of RNA sequences, the result was a greater number of hybridized sites on the gene chip.

She tested the concept on human infants who had undergone a cardiac transplant. Young children have a large thymus — the organ at the base of the neck that’s key to developing immune system cells, particularly T-cells — and as part of the transplant procedure, doctors remove it. In addition, the infants scheduled for cardiac transplant undergo T-cell depletion therapy prior to the procedure to avoid organ rejection.

“So they should, in principle, have a very low T-cell diversity,” says Ogle.

Her method is now patented and she and others are working with the National Institutes of Health to use it as a diagnostic tool for patients with various immune disorders. “They’re planning to use the assay for some of their HIV patients who have normal CD4 T-cell counts but have recurrent viral infections,” she says. “They suspect the infections could reflect a deficiency in T-cell diversity — not T-cell number. If this were the case, therapies could be adjusted to improve the outcome.”

Once the pig-as-surrogate-host model (the focus of the transplant biology group’s research) was established, the researchers began to probe the newborn pigs for human T-cells and found that 60 percent of those they’d injected contained a large fraction of the human cells.

Using Ogle’s new measurement method, the group tested T-cell diversity and learned that the cells had been generated in the pig’s thymus, and that they could respond to a foreign antigen.

“Ultimately, we would take bone marrow from an individual suffering from a T-cell defect, expand their T-cells in a pig, and take those new human T-cells and put them back in the individual to determine whether or not they will function therapeutically,” she says.

The pig-as-surrogate study also yielded some surprising results: Ogle found that during the time that human T-cells resided in the pigs, some of those cells fused with pig cells.

“We found that those few cells actually were maintained long term and were the primary cells that contributed to tissues — not just T-cells, but other tissues in pigs,” she says.

As she begins her research program in Wisconsin, Ogle will study specifically how cell fusion might direct stem cell differentiation, and in particular, cardiomyocyte differentiation and whether those cells could be used to enhance regeneration in the heart.

“It all ties back into immune responses, because one of the problems that we’re now beginning to realize is that immune responses are elicited against stem cells,” she says. “A large challenge will be overcoming or circumventing immune responses to stem cells while maintaining their therapeutic benefit.”

Ogle anticipates collaborating with Wisconsin embryonic stem cell pioneers, geneticists, virologists, materials scientists, mechanical engineers and others to develop testable solutions they can transform into clinical practice.